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Title:
ELECTRIC MACHINE APPARATUS
Document Type and Number:
WIPO Patent Application WO/2018/099811
Kind Code:
A1
Abstract:
The present disclosure relates to a rotor (3) for an electric machine (1). The rotor (3) includes a plurality of permanent magnets (6-n), each permanent magnet being mounted in a magnet aperture (10-n) formed in the rotor (3). A plurality of flux barriers (11-1, 11-2, 12-1, 12-2, 23-1, 23-2) and bridges (13-1, 13-2, 14-1, 14-2, 24-1) are formed in the rotor (3). The bridges (13-1, 13-2, 14-1, 14-2, 24-1) are each formed between one of said magnet apertures (10-n) and an adjacent one of said flux barriers (11-1, 11-2, 12-1, 12-2, 23-1, 23- 2). Each bridge comprises opposing first and second sides (15-1, 15-2, 16-1) formed by the magnet aperture (10-n) and the flux barrier respectively. The first side includes a first circular arc (17B, 21B, 27B, 33B, 37B) (17B, 21B, 27B) and the second side includes a second circular arc (18B, 22B, 28B, 34B, 38B) (18B, 22B, 28B). The present disclosure also relates to an electric machine (1) including a rotor (3); and to a vehicle (2) including an electric machine (1).

Inventors:
BONYADI ROOZBEH (GB)
MICHAELIDES ALEXANDROS (GB)
KIRALY ISTVAN (GB)
CESA TIAGO (GB)
Application Number:
PCT/EP2017/080293
Publication Date:
June 07, 2018
Filing Date:
November 24, 2017
Export Citation:
Click for automatic bibliography generation   Help
Assignee:
JAGUAR LAND ROVER LTD (GB)
International Classes:
H02K1/27
Domestic Patent References:
WO2015097767A12015-07-02
Foreign References:
JP2009112181A2009-05-21
EP2600496A12013-06-05
US20140077650A12014-03-20
US20130119817A12013-05-16
US20150357872A12015-12-10
US20140217849A12014-08-07
Other References:
None
Attorney, Agent or Firm:
ELLIS, Richard (GB)
Download PDF:
Claims:
CLAIMS:

1 . A rotor (3) for an electric machine (1 ), the rotor (3) comprising:

a plurality of permanent magnets (6-n), each permanent magnet being mounted in a magnet aperture (10-n) formed in the rotor (3);

a plurality of flux barriers (1 1 -1 , 1 1 -2, 12-1 , 12-2, 23-1 , 23-2) formed in the rotor (3); and

a plurality of bridges (13-1 , 13-2, 14-1 , 14-2, 24-1 ), each bridge being formed in the rotor (3) between one of said magnet apertures (10-n) and an adjacent one of said flux barriers (1 1 -1 , 1 1 -2, 12-1 , 12-2, 23-1 , 23-2);

wherein each bridge comprises opposing first and second sides (15-1 , 15-2, 16-1 ) formed by the magnet aperture (10-n) and the flux barrier respectively, the first side comprising a first circular arc (17B, 21 B, 27B, 33B, 37B) and the second side comprising a second circular arc (18B, 22B, 28B, 34B, 38B).

2. A rotor (3) as claimed in claim 1 , wherein the first circular arc (17B, 21 B, 27B, 33B, 37B) has substantially the same radius as the second circular arc (18B, 22B, 28B, 34B, 38B). 3. A rotor (3) as claimed in claim 1 or claim 2, wherein the first circular arc (17B, 21 B, 27B, 33B, 37B) has substantially the same length as the second circular arc (18B, 22B, 28B, 34B, 38B).

4. A rotor (3) as claimed in any one of claims 1 , 2 or 3, wherein a first virtual chord line extends between the ends of the first circular arc (17B, 21 B, 27B, 33B, 37B) and a second virtual chord line extends between the ends of the second circular arc (18B, 22B, 28B, 34B, 38B).

5. A rotor (3) as claimed in claim 4, wherein the first and second circular arcs (17B, 18B, 21 B, 22B, 27B, 28B, 33B, 34B, 37B, 38B) are arranged such that said first and second virtual chord lines are disposed substantially parallel to each other.

6. A rotor (3) as claimed in claim 4, wherein the first and second circular arcs (17B, 18B, 21 B, 22B, 27B, 28B, 33B, 34B, 37B, 38B) are arranged such that said first and second virtual chord lines are oriented at a chord line angle (a1 , a2) relative to each other.

7. A rotor (3) as claimed in claim 6, wherein said chord line angle (a1 , a2) is less than or equal to 10° 15° or 20°.

8. A rotor (3) as claimed in any one of the preceding claims, wherein the first side of each bridge comprises a second circular arc (17A, 21 A, 27A, 33A, 37A) and a third circular arc (17C, 21 C, 27C, 33C, 37C), the first circular arc (17B, 21 B, 27B, 33B, 37B) being disposed between said second and third circular arcs.

9. A rotor (3) as claimed in claim 8, wherein said first, second and third circular arcs (17A, 17B, 17C, 21 A, 21 B, 21 C, 27A, 27B, 27C, 33A, 33B, 33C, 37A, 37B, 37C) are arranged to form a substantially continuous curved profile.

10. A rotor (3) as claimed in any one of the preceding claims, wherein the second side of each bridge comprises a second circular arc (17A, 21 A, 27A, 33A, 37A) and a third circular arc (17C, 21 C, 27C, 33C, 37C), the first circular arc (17B, 21 B, 27B, 33B, 37B) being disposed between said second and third circular arcs.

1 1 . A rotor (3) as claimed in claim 10, wherein said first, second and third circular arcs (17A, 17B, 17C, 21 A, 21 B, 21 C, 27A, 27B, 27C, 33A, 33B, 33C, 37A, 37B, 37C) form a substantially continuous curved profile.

12. A rotor (3) for an electric machine (1 ), the rotor (3) comprising:

a plurality of permanent magnets (6-n), each permanent magnet being mounted in a magnet aperture (10-n) formed in the rotor (3);

a plurality of flux barriers (1 1 -1 , 1 1 -2, 12-1 , 12-2, 23-1 , 23-2) formed in the rotor (3); and

a plurality of bridges (13-1 , 13-2, 14-1 , 14-2, 24-1 ), each bridge being formed in the rotor (3) between one of said magnet apertures (10-n) and an adjacent one of said flux barriers (1 1 -1 , 1 1 -2, 12-1 , 12-2, 23-1 , 23-2); each bridge (13-1 , 13-2, 14-1 , 14-2, 24-1 ) comprising opposing first and second sides (15-1 , 15-2, 16-1 ) formed by the magnet aperture (10-n) and the flux barrier respectively;

wherein the first side of each bridge (13-1 , 13-2, 14-1 , 14-2, 24-1 ) comprises a plurality of circular arcs arranged to form a substantially continuous curved profile; and/or the second side of each bridge (13-1 , 13-2, 14-1 , 14-2, 24-1 ) comprises a plurality of circular arcs arranged to form a substantially continuous curved profile.

13. A rotor (3) as claimed in claim 12, wherein the first side of each bridge (13-1, 13-2, 14-1, 14-2, 24-1 ) consists of three circular arcs.

14. A rotor (3) as claimed in claim 12 or claim 13, wherein the second side of each bridge consists of three circular arcs.

15. A rotor (3) as claimed in any one of the preceding claims, wherein the first and second sides (15-1, 15-2, 16-1) of the bridge are concave such that the bridge has a biconcave shape.

16. A rotor (3) as claimed in any one of the preceding claims comprising a plurality of magnet poles (5a-h), the magnet poles (5a-h) each comprising at least a first layer (L3) including one or more of said permanent magnets (6-n). 17. A rotor (3) as claimed in claim 16, wherein one of said flux barriers (11-1, 11-2, 12- 1 , 12-2, 23-1 , 23-2) (12-1, 12-2) is formed on each side of the first layer (L3).

18. A rotor (3) as claimed in claim 16 or claim 17, wherein the magnet poles (5a-h) each comprise a second layer (L2) including one or more of said permanent magnets (6-n).

19. A rotor (3) as claimed in claim 18, wherein one of said flux barriers (11-1, 11-2, 12- 1 , 12-2, 23-1 , 23-2) is formed on each side of the second layer (L2).

20. A rotor (3) as claimed in any one of claims 16 to 19, wherein the magnet poles (5a- h) each comprise a third layer (L1 ) including one or more of said permanent magnets (6-n).

21. A rotor (3) as claimed in any one of the preceding claims, wherein the flux barriers (11-1, 11-2, 12-1, 12-2, 23-1, 23-2) comprise one or more inner flux barriers (11-1, 11-2); and/or one or more outer flux barriers (12-1, 12-2).

22. A rotor (3) as claimed in any one of the preceding claims, comprising at least one interposed flux barrier (30-1, 30-2, 49-1, 39-2, 40-1, 40-2) disposed between one of said magnet apertures and an adjacent one of said flux barriers (11-1, 11-2, 12-1, 12-2, 23-1, 23- 2).

23. A rotor for an electric machine, the rotor comprising: a plurality of permanent magnets, each permanent magnet being mounted in a magnet aperture formed in the rotor;

a plurality of flux barriers (11-1, 11-2, 12-1, 12-2, 23-1, 23-2) formed in the rotor; and

a plurality of bridges (13-1, 13-2, 14-1, 14-2, 24-1);

wherein at least one interposed flux barrier (30-1 , 30-2, 49-1 , 39-2, 40-1 , 40-2) is interposed between one of said magnet apertures and an adjacent one of said flux barriers (11-1, 11-2, 12-1, 12-2, 23-1, 23-2). 24. A rotor (3) as claimed in claim 22 or claim 23, wherein said at least one interposed flux barrier (30-1, 30-2, 49-1, 39-2, 40-1, 40-2) form a plurality of said bridges (13-1, 13-2, 14-1, 14-2, 24-1) between one of said magnet apertures and an adjacent one of said flux barriers (11-1, 11-2, 12-1, 12-2, 23-1 , 23-2). 25. A rotor (3) as claimed in any one of claims 22, 23 or 24, wherein said at least one interposed flux barrier (30-1, 30-2, 49-1, 39-2, 40-1, 40-2) comprise at least one interposed flux barrier (30-1 , 30-2, 49-1 , 39-2, 40-1 , 40-2) having a closed perimeter.

26. A rotor (3) as claimed in any one of claims 22 to 25, wherein said at least one interposed flux barrier (30-1, 30-2, 49-1, 39-2, 40-1, 40-2) comprise at least one interposed flux barrier (30-1 , 30-2, 49-1 , 39-2, 40-1 , 40-2) having an open perimeter.

27. A rotor (3) as claimed in any one of claims 22 to 26, wherein said at least one interposed flux barrier (30-1 , 30-2, 49-1 , 39-2, 40-1 , 40-2) comprise a void having a teardrop profile.

28. A rotor (3) as claimed in any one of the preceding claims, wherein at least one of said bridges (13-1, 13-2, 14-1, 14-2, 24-1) comprising first and second sides (15-1, 15-2, 16- 1) comprising opposing first and second arcs having respective first and second virtual cords, wherein said first and second virtual cords are inclined at a chord line angle relative to each other.

29. A rotor (3) as claimed in claim 28, wherein said chord line angle is less than or equal to one of the following set: 10°, 8°, 6°, 4° or 2°.

30. An electric machine (1) comprising a rotor (3) as claimed in any one of the preceding claims.

31 . An electric machine (1 ) as claimed in claim 30, wherein the electric machine (1 ) is a permanent magnet synchronous machine.

32. A vehicle (2) comprising an electric machine (1 ) as claimed in claim 30 or claim 31 .

Description:
ELECTRIC MACHINE APPARATUS

TECHNICAL FIELD

The present disclosure relates to an electric machine apparatus. Particularly, but not exclusively, the present disclosure relates to a rotor for an electric machine; to an electric machine comprising a rotor; and to a vehicle comprising an electric machine as claimed in the appended claims.

BACKGROUND

A particular design challenge for electric machines is the design of the rotor to reduce the mechanical stresses induced within the rotor as it rotates. The rotor comprises a plurality of permanent magnets mounted in apertures formed therein. The rotor may also comprise flux barriers in the form of apertures for controlling the magnetic flux generated by the permanent magnets. The magnet apertures and flux barriers form a plurality of bridges (or ligaments) within the rotor. When the electric machine is operating, mechanical stresses are induced within the rotor as it rotates. The resulting mechanical stresses are concentrated in the bridges within the rotor. The bridges within the rotor must be sufficiently strong to withstand the resulting structural loads. At least in certain embodiments the present invention seeks to configure the rotor to reduce mechanical stresses.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to a rotor for an electric machine; to an electric machine comprising a rotor; and to a vehicle as claimed in the appended claims. According to a further aspect of the present invention there is provided a rotor for an electric machine, the rotor comprising:

a plurality of permanent magnets, each permanent magnet being mounted in a magnet aperture formed in the rotor;

a plurality of flux barriers formed in the rotor; and

a plurality of bridges, each bridge being formed in the rotor between one of said magnet apertures and an adjacent one of said flux barriers;

wherein each bridge comprises opposing first and second sides formed by the magnet aperture and the flux barrier respectively, the first side comprising a first circular arc and the second side comprising a second circular arc. At least in certain embodiments this arrangement of the first and second sides of each bridge may help to distribute stresses more evenly within the bridges. At least in certain embodiments, the bridges may be configured to maintain mechanical integrity of the rotor at high rotational speeds. The bridges may also be effective in controlling flux density in the rotor. The bridges may be configured to define a reluctance flux path within the rotor.

The first circular arc may have a smaller radius than the second circular arc. Alternatively, the first circular arc may have a larger radius than the second circular arc. At least in certain embodiments, the first circular arc may have substantially the same radius as the second circular arc.

The first circular arc may have a length greater than or less than a length of the second circular arc. At least in certain embodiments the first circular arc may have substantially the same length as the second circular arc.

A first virtual chord line may be defined between the ends of the first circular arc and a second virtual chord line may be defined between the ends of the second circular arc. The first and second virtual chord lines may be defined in a transverse section through said rotor (i.e. perpendicular to a rotational axis of the rotor).

The first and second circular arcs may be arranged such that said first and second virtual chord lines are disposed substantially parallel to each other.

The first and second circular arcs may be arranged such that said first and second virtual chord lines are oriented at a chord line angle relative to each other. The chord line angle may be greater than or equal to 0.1 °. The chord line angle may be less than or equal to 10°, 15° or 20°. The first and second virtual chord lines may converge or diverge in a first direction. The chord line angle may be positive or negative.

The plurality of flux barriers may comprise internal and/or external flux barriers. The internal flux barriers are apertures formed within the rotor. The external flux barriers are apertures formed at the periphery of the rotor, for example in an external surface of the rotor.

The first side of each bridge may comprise more than one circular arc. The first side of each bridge may comprise a plurality of circular arcs. The ends of the circular arcs may be connected to each other. The circular arcs may be arranged such that the first side of each bridge comprises or consists of a substantially continuous curve. The first side of each bridge may comprise a second circular arc. The first side of each bridge may comprise a third circular arc. The second circular arc may have substantially the same radius as the third circular arc. Alternatively, the second circular arc may have a different radius than the third circular arc. The first circular arc may have a radius which is different from the radius of the second circular arc and/or said third circular arc. The first circular arc may have a radius which is larger than the radius of the second circular arc and/or said third circular arc. The first circular arc may be disposed between said second and third circular arcs. The first, second and third circular arcs may be arranged to form a substantially continuous curved profile. The first, second and third circular arcs may be conjoined. One of said second and third circular arcs may be extended to form a teardrop shape in the magnet aperture.

The second side of each bridge may comprise more than one circular arc. The second side of each bridge may comprise a plurality of circular arcs. The ends of the circular arcs may be connected to each other. The circular arcs may be arranged such that the second side of each bridge comprises or consists of a substantially continuous curve. The second side of each bridge may comprise a second circular arc. The second side of each bridge may comprise a third circular arc. The second circular arc may have substantially the same radius as the third circular arc. Alternatively, the second circular arc may have a different radius than the third circular arc. The first circular arc may have a radius which is different from the radius of the second circular arc and/or said third circular arc. The first circular arc may have a radius which is larger than the radius of the second circular arc and/or said third circular arc. The first circular arc may be disposed between said second and third circular arcs. The first, second and third circular arcs may form a substantially continuous curved profile. The first, second and third circular arcs may be conjoined. One of said second and third circular arcs may be extended to form an inverted teardrop shape in the flux barrier.

According to a further aspect of the present invention there is provided a rotor for an electric machine, the rotor comprising:

a plurality of permanent magnets, each permanent magnet being mounted in a magnet aperture formed in the rotor;

a plurality of flux barriers formed in the rotor; and

a plurality of bridges, each bridge being formed in the rotor between one of said magnet apertures and an adjacent one of said flux barriers; each bridge comprising opposing first and second sides formed by the magnet aperture and the flux barrier respectively;

wherein the first side of each bridge comprises a plurality of circular arcs arranged to form a substantially continuous curved profile; and/or the second side of each bridge comprises a plurality of circular arcs arranged to form a substantially continuous curved profile. The first side of each bridge comprises a plurality of circular arcs. The ends of the circular arcs may be connected to each other. The circular arcs may be arranged such that the first side of each bridge comprises or consists of a substantially continuous curve. The circular arcs may be arranged such that the first side of each bridge is concave.

The first side of each bridge may consist of three circular arcs. The three circular arcs may be conjoined. One of said circular arcs may extend to form a teardrop feature in a lower portion of the magnet aperture. The second side of each bridge comprises a plurality of circular arcs. The ends of the circular arcs may be connected to each other. The circular arcs may be arranged such that the second side of each bridge comprises or consists of a substantially continuous curve. The circular arcs may be arranged such that the second side of each bridge is concave. The second side of each bridge may consist of three circular arcs. The three circular arcs may be conjoined. One of said circular arcs may extend to form an inverted teardrop feature in an upper portion of the flux barrier.

The first and second sides of the bridge may be concave such that the bridge has a biconcave shape. A middle section of the bridge is narrower than one or both ends of the bridge.

The rotor may comprise a plurality of magnet poles. The magnet poles may each comprise at least a first layer including one or more of said permanent magnets. One of said flux barriers may be formed on each side of the first layer.

The magnet poles may each comprise a second layer including one or more of said permanent magnets. One of said flux barriers may be formed on each side of the second layer.

The magnet poles may each comprise a third layer including one or more of said permanent magnets.

The flux barriers are provided to control flux density distribution within the rotor. The flux barriers may each comprise an aperture formed in the rotor. The aperture may comprise an internal aperture or an external aperture. The rotor may comprise one or more inner flux barriers; and/or one or more outer flux barriers. The terms "inner" and "outer" may be used to define the location of the flux barriers relative to a pole axis (d-axis) of an associated magnet pole. The outer flux barriers may be disposed laterally outside the inner flux barriers. The rotor may comprise first and second inner barriers. The rotor may comprise first and second outer flux barriers.

The rotor may comprise at least one interposed flux barrier disposed between one of said magnet apertures and an adjacent one of said flux barriers. The incorporation of at least one interposed flux barrier in the rotor is believed to be independently patentable.

According to a further aspect of the present invention there is provided a rotor for an electric machine, the rotor comprising:

a plurality of permanent magnets, each permanent magnet being mounted in a magnet aperture formed in the rotor;

a plurality of flux barriers formed in the rotor; and

a plurality of bridges;

wherein at least one interposed flux barrier is interposed between one of said magnet apertures and an adjacent one of said flux barriers. The at least one interposed flux barrier may form a plurality of said bridges between one of said magnet apertures and an adjacent one of said flux barriers. By way of example, the rotor may comprise one of said interposed flux barriers interposed between one of said magnet apertures and an adjacent one of said flux barriers to form two (2) of said bridges. By way of further example, the rotor may comprise two of said interposed flux barriers interposed between one of said magnet apertures and an adjacent one of said flux barriers to form three (3) of said bridges. Three or more interposed flux barriers may be provided between said magnet aperture and the adjacent flux barrier. It will be appreciated that different numbers of said interposed flux barriers may be provided between said magnet aperture and the adjacent flux barrier in different layers of the rotor.

One of said interposed flux barriers may be interposed between said magnet aperture and the adjacent flux barrier. Alternatively, more than one of said interposed flux barriers may be interposed between said magnet aperture and the adjacent flux barrier. The at least one interposed flux barrier may comprise at least one interposed flux barrier having a closed perimeter. The at least one interposed flux barrier may be surrounded by the material forming the rotor. The at least one interposed flux barrier may be inset from an inner edge or outer edge of the rotor such that said inner edge or said outer edge is at least substantially continuous.

The at least one interposed flux barrier may comprise at least one interposed flux barrier having an open perimeter. The at least one interposed flux barrier may be open to an inner edge or an outer edge of the rotor such that said inner edge or said outer edge is interrupted or discontinuous.

The at least one interposed flux barrier may have a profile comprising or consisting of a plurality of circular arcs. The circular arcs may have different radii and/or different centres. The circular arcs may be arranged such that the at least one interposed flux barrier has a substantially continuous outer profile. The circular arcs may blend together to form a surface free from apexes or other sudden direction changes. Thus, the at least one interposed flux barrier may have a smooth outer profile.

The at least one interposed flux barrier may comprise a void having a teardrop profile.

The at least one interposed flux barrier may each have a major axis and a minor axis. The at least one interposed flux barrier may be symmetrical about one or both of the major axis and the minor axis. The rotor may comprise a plurality of said interposed flux barriers associated at least first and second permanent magnets arranged in first and second layers. The interposed flux barriers may be arranged such that at least some of said major axes are arranged substantially parallel to each other. At least one of said bridges may comprise first and second sides comprising opposing first and second arcs having respective first and second virtual chords. The first and second virtual chords may be inclined at a chord line angle relative to each other. The chord line angle may be non-zero. The chord line angle may, for example, be in the range 0 °° to 10°. The chord line angle may be less than or equal to one of the following set: 10°, 8°, 6°, 4° or 2°. Alternatively, the first and second virtual chords may be substantially parallel to each other, equivalent to a chord line angle of zero.

According to a further aspect of the present invention there is provided an electric machine comprising a rotor as described herein. According to a further aspect of the present invention there is provided an electric machine as described herein. The electric machine may be a permanent magnet synchronous machine. According to a further aspect of the present invention there is provided a vehicle comprising an electric machine as described herein.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which:

Figure 1 shows a schematic representation of a vehicle incorporating an electric machine in accordance with an embodiment of the present invention;

Figure 2 shows a transverse section through the electric machine shown in Figure

1 ;

Figure 3A shows a segment of the rotor of the electric machine shown in Figure 1 ;

Figure 3B shows an enlarged view of a bridge formed between a magnet aperture in an inner layer and an outer flux barrier of the rotor shown in Figure 3A;

Figure 3C shows an enlarged view of a bridge formed between a magnet aperture in an intermediate layer and an inner flux barrier of the rotor shown in Figure 3A;

Figure 4A shows a segment of a rotor of an electric machine according to a further embodiment of the present invention;

Figure 4B shows an enlarged view of a bridge formed between a magnet aperture in an inner layer and an outer flux barrier of the rotor shown in Figure 4A;

Figure 4C shows an enlarged view of a bridge formed between a magnet aperture in an intermediate layer and an inner flux barrier of the rotor shown in Figure 4A;

Figure 5A shows a variant of the segment shown in Figures 4A-C; Figure 5B shows an enlarged view of a bridge formed between a magnet aperture in an outer layer and an external groove formed in an outer surface of the rotor;

Figure 6A shows a segment of a rotor of an electric machine according to a further embodiment of the present invention;

Figure 6B shows an enlarged view of a bridge formed between a magnet aperture in an inner layer and an outer flux barrier of the rotor shown in Figure 5A;

Figure 7A shows a segment of a rotor of an electric machine according to a further embodiment of the present invention;

Figure 7B shows an enlarged view of an interposed flux barrier forming a plurality of bridges between a magnet aperture in an inner layer and an outer flux barrier of the rotor shown in Figure 7A;

Figure 7C shows a variant of the interposed flux barrier shown in Figures 7A and

7B;

Figure 8A shows a segment of a rotor of an electric machine according to a further embodiment of the present invention;

Figure 8B shows an enlarged view of an interposed flux barrier formed between a magnet aperture in an inner layer and an outer flux barrier of the rotor shown in Figure 8A; and

Figure 8C shows an enlarged view of an interposed flux barrier formed between a magnet aperture in an intermediate layer and an inner flux barrier of the rotor shown in Figure 8A.

DETAILED DESCRIPTION

An electric machine 1 in accordance with an embodiment of the present invention will now be described. The electric machine 1 in the present embodiment is configured for use as a traction drive in a motor vehicle 2, as shown schematically in Figure 1 .

With reference to Figure 2, the electric machine 1 is a permanent magnet synchronous motor comprising a rotor 3 and a stator 4. An air gap is maintained between the rotor 3 and the stator 4. The rotor 3 is made up of a plurality of laminations of a ferromagnetic material to form a rotor iron. The rotor 3 is configured to rotate about a rotational axis Z (extending perpendicular to the plane of the page in Figure 2). The rotor 3 comprises eight (8) magnet poles 5a-h each comprising three (3) permanent magnets 6-n (where n represents the number of magnets in each of said magnet poles 5a-h). In alternate embodiments, the rotor 3 may comprise less than or more than eight (8) magnet poles 5a-h. Moreover, each magnet pole 5a-h may comprise less than or more than three (3) magnets 6-n. The stator 4 comprises a plurality of slots 8 extending radially inwardly to support coil windings 9. In the present arrangement, the stator 4 comprises forty-eight (48) slots 8 such that there are six (6) slots 8 for each magnet pole 5. The permanent magnets 6-n generate a magnetic flux and a torque is generated to drive the rotor 3 by energising the coil winding 9. The magnet poles 5a-h are angularly separated from each other and an inter-pole region 7a- h is formed between adjacent magnet poles 5a-h. The magnet poles 5a-h each extend radially outwardly from the rotational axis Z of the rotor 3. The rotor 3 has an outer surface SC1 in the form of a circular cylinder having a radius R. The magnet poles 5a-h all have the same general configuration and, for the sake of brevity, only a first magnet pole 5a will be described herein. An assumed reference frame for the first magnet pole 5a is shown in Figure 3A. The reference frame comprises a pole axis (d-axis) aligned to the permanent magnet flux of the first magnet pole 5a. An inter-pole axis (q-axis) arranged transverse to the direction of the first magnet pole 5a (i.e. transverse to the pole axis (d-axis)) forms a centreline of the inter-pole regions 7a-h. The angular separation of the d-axes of adjacent magnet poles 5a-h is 45°. The angular separation of the pole axis (d-axis) of the first magnet pole 5a and the inter-pole axis (q-axis) of an adjacent first inter-pole region 7a is 30° in the present embodiment.

The permanent magnets 6-n are each mounted in a respective magnet aperture 10-n formed in the rotor 3. The magnet apertures 10-n are internal apertures which extend substantially parallel to the rotational axis Z. The permanent magnets 6-n in the first magnet pole 5a are arranged in a radially outer layer L1 , an intermediate layer L2 and a radially inner layer L3. As shown in Figure 2, the outer, intermediate and inner layers L1 -L3 are arranged concentrically about the rotational axis Z of the rotor 3 with a radial offset between each of the outer, intermediate and inner layers L1 -L3. The outer, intermediate and inner layers L1 - L3 within the first pole 5a each comprise one of said permanent magnets 6-n. The permanent magnets 6-n are each arranged substantially perpendicular to the radius R of the rotor 3. In an alternative embodiment, two or more permanent magnets 6-n may be provided in each of the outer, intermediate and inner layers L1 -L3. In alternative embodiments the permanent magnets 6-n may be arranged in only some of said outer, intermediate and inner layers L1 -L3. For example, the magnet poles 5a-h may comprise an outer layer L1 and/or an inner layer L3. In certain embodiments, the permanent magnets 6-n may be arranged in additional layers. The magnet apertures 10-n each comprise first and second lateral stops S- 1 , S-2 for inhibiting lateral movement of the permanent magnets 6-n. The first and second lateral stops S-1 , S-2 comprise first and second protrusions for engaging respective sides of the permanent magnet 6-n disposed in said magnet apertures 10-n. First and second inner flux barriers 1 1 -1 , 1 1 -2; and first and second outer flux barriers 12-1 , 12-2 are associated with each of the magnet poles 5a-h. The terms "inner" and "outer" are used to define the location of the flux barriers relative to the pole axis (d-axis) of the first magnet pole 5a. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are provided to help ensure the appropriate flux density distribution at the lateral boundaries of the magnet poles 5a-h. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12- 2 are arranged symmetrically about the pole axis (d-axis). The inner flux barriers 1 1 -1 , 1 1 -2 are disposed on opposing sides of the permanent magnet 6-2 in the intermediate layer L2; and the outer flux barriers 12-1 , 12-2 are disposed on opposing sides of the permanent magnet 6-1 in the inner layer L3. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 each comprise an air-filled cavity elongated in a radial direction from the centre of the rotor 3. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are internal apertures (or holes) formed within the rotor 3 and are inset from the outer surface SC1 of the rotor 3. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are surrounded by the ferromagnetic material of the rotor 3. Thus, the inner flux barriers 1 1 - 1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 have a closed perimeter.

The outer flux barriers 12-1 , 12-2 are associated with the magnet aperture 10-1 in the inner layer L3. The inner flux barriers 1 1 -1 , 1 1 -2 are associated with the intermediate magnet aperture 10-2 in the intermediate layer L2. First and second inner bridges 13-1 , 13-2 are formed between the magnet aperture 10-1 in the inner layer L3 and the outer flux barriers 12-1 , 12-2. First and second intermediate bridges 14-1 , 14-2 are formed between the intermediate magnet aperture 10-2 in the intermediate layer L2 and the inner flux barriers 1 1 -1 , 1 1 -2. The configuration of the first and second inner bridges 13-1 , 13-2 and the first and second intermediate bridges 14-1 , 14-2 will now be described in more detail.

As shown in Figure 3A, the first and second inner bridges 13-1 , 13-2 are symmetrical about the pole axis (d-axis) and have the same configuration. The configuration of the first inner bridge 13-1 will now be described with reference to Figure 3B. The first inner bridge 13-1 comprises an inner lateral side 15-1 defined by the magnet aperture 10-1 ; and an outer lateral side 16-1 defined by the first outer flux barrier 12-1 . The inner lateral side 15-1 and the outer lateral side 16-1 form opposing sides of the first inner bridge 13-1 and are concave such that the first inner bridge 13-1 has a biconcave profile. The inner lateral side 15-1 comprises top, middle and bottom inner arcs 17A, 17B, 17C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 17A, 17B, 17C are circular arcs. The outer lateral side 16-1 comprises top, middle and bottom outer arcs 18A, 18B, 18C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 18A, 18B, 18C are circular arcs. In the present embodiment the top outer arc 18A forms an inverted teardrop feature in the first outer flux barrier 12-1 .

The middle inner and outer arcs 17B, 18B have at least substantially the same radius and at least substantially the same length. The relative position and orientation of the middle inner arc 17B and the middle outer arc 18B affect the mechanical stress induced in the first inner bridge 13-1 during operation of the electric machine 1 . The orientation of the middle inner arc 17B is described with reference to an inner virtual chord C1 ; and the orientation of the middle outer arc 18B is described with reference to an outer virtual chord C2. The inner virtual chord C1 extends between the ends of the middle inner arc 17B; and the outer virtual chord C2 extends between the ends of the middle outer arc 18B. The relative position and orientation of the first inner and outer virtual chords C1 , C2 are determined by the relative position of the centres of the middle inner and outer arcs 17B, 18B. The first inner and outer virtual chords C1 , C2 are inclined at a first chord line angle a1 relative to each other. The first inner and outer virtual chords C1 , C2 diverge from each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the first chord line angle a1 between the first inner and outer virtual chords C1 , C2 distributes mechanical stress more evenly within the first inner bridge 13-1 . The first chord line angle a1 in the first inner bridge 13-1 is approximately 8.61 0 in the present embodiment. A first centreline CL1 of the first inner bridge 13-1 is defined between said first inner and outer virtual chords C1 , C2. The first centreline CL1 is inclined at a first centreline angle relative to the radius R of the rotor 3. The arrangement of the first inner bridge 13-1 is mirrored about the pole axis (d-axis) for the second inner bridge 13-2. As shown in Figure 3A, the first and second intermediate bridges 14-1 , 14-2 are symmetrical about the pole axis (d-axis) and have the same configuration. The configuration of said first intermediate bridge 14-1 will now be described with reference to Figure 3C. The first intermediate bridge 14-1 comprises an inner lateral side 19-1 defined by the intermediate magnet aperture 10-2; and an outer lateral side 20-1 defined by the first inner flux barrier 1 1 - 1 . The inner lateral side 19-1 and the outer lateral side 20-1 are concave such that the first intermediate bridge 14-1 has a biconcave profile. The inner lateral side 19-1 comprises top, middle and bottom inner arcs 21 A, 21 B, 21 C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 21 A, 21 B, 21 C are circular arcs. The outer lateral side 20-1 comprises top, middle and bottom outer arcs 22A, 22B, 22C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 22A, 22B, 22C are circular arcs. The middle inner and outer arcs 21 B, 22B have at least substantially the same radius and at least substantially the same length. The relative position and orientation of the second middle inner arc 21 B and the second middle outer arc 22B affect the mechanical stress in the first intermediate bridge 14-1 when the electric machine 1 is operating. The orientation of the second middle inner arc 21 B is described with reference to a virtual second inner virtual chord C3; and the orientation of the second middle outer arc 22B is described with reference to an outer virtual chord C4. The second inner virtual chord C3 extends between the ends of the second middle inner arc 21 B; and the outer virtual chord C4 extends between the ends of the second middle outer arc 22B. The relative position and orientation of the second inner and outer virtual chords C3, C4 are determined by the relative position of the centres of the middle inner and outer arcs 21 B, 22B. The second inner and outer virtual chords C3, C4 are inclined at a second chord line angle a2 relative to each other. The second inner and outer virtual chords C3, C4 diverge from each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the second chord line angle a2 between the second inner and outer virtual chords C3, C4 distributes mechanical stress more evenly within the first intermediate bridge 14-1 . The second chord line angle a2 in the first intermediate bridge 14-1 is approximately 7.52 ° in the present embodiment. A second centreline CL2 of the first intermediate bridge 14-1 is defined between said second inner and outer chords C3, C4. The second centreline CL2 is inclined at a second centreline angle β2 relative to the radius R of the rotor 3. The arrangement of the first intermediate bridge 14-1 is mirrored about the pole axis (d-axis) for the second intermediate bridge 14-2.

The first and second inner bridges 13-1 , 13-2 and the first and second intermediate bridges 14-1 , 14-2 are configured to maintain the mechanical integrity of the laminations of the rotor 3 at high rotational speeds whilst also controlling flux density. In the present embodiment, there are no flux barriers associated with the outer layer L1 . It will be understood that in a variant, additional flux barriers may be provided. For example, flux barriers may be associated with the outer magnet aperture 10-3 in the outer layer L1 . A pair of third bridges (not shown) may be formed between the outer magnet aperture 10-3 and the additional flux barriers. In a further variant, one or more of the flux barriers may be an external flux barrier adapted to extend outwardly to a sidewall of the rotor 3.

A further embodiment of the rotor 3 in accordance with an aspect of the present invention will now be described with reference to Figures 4A-C. Like reference numerals are used for like components. The description of this embodiment will focus on the differences over the embodiment shown in Figures 3A-C. The rotor 3 comprises six (6) magnet poles 5a-h each comprising three (3) permanent magnets 6-n (where n represents the number of magnets in each of said magnet poles 5a-h) arranged in an outer layer L1 , an intermediate layer L2 and an inner layer L3. The permanent magnets 6-n are mounted in magnet apertures 10-n. The rotor 3 comprises inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 each comprise an air-filled cavity. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are internal apertures (or holes) formed within the rotor 3 and are inset from the outer surface SC1 of the rotor 3. First and second external flux barriers 23-1 , 23-2 are formed in the outer surface of the rotor 3.

The outer flux barriers 12-1 , 12-2 are associated with the magnet aperture 10-1 in the inner layer L3. First and second inner bridges 13-1 , 13-2 are formed between the magnet aperture

10- 1 in the inner layer L3 and the outer flux barriers 12-1 , 12-2. The inner flux barriers 1 1 -1 ,

1 1 - 2 are associated with the intermediate magnet aperture 10-2 in the intermediate layer L2. First and second intermediate bridges 14-1 , 14-2 are formed between the intermediate magnet aperture 10-2 in the intermediate layer L2 and the inner flux barriers 1 1 -1 , 1 1 -2. The first and second external flux barriers 23-1 , 23-2 are associated with the outer magnet aperture 10-3 in the outer layer L1 . First and second outer bridges 24-1 , 24-2 are formed between the outer magnet aperture 10-3 in the outer layer L1 and the first and second external flux barriers 23-1 , 23-2.

As shown in Figure 4A, the first and second inner bridges 13-1 , 13-2 are symmetrical about the pole axis (d-axis) and have the same configuration. The configuration of said first inner bridge 13-1 will now be described with reference to Figure 4B. The first inner bridge 13-1 comprises an inner lateral side 15-1 defined by the magnet aperture 10-1 ; and an outer lateral side 16-1 defined by the first outer flux barrier 12-1 . The inner lateral side 15-1 and the outer lateral side 16-1 are concave such that the first inner bridge 13-1 has a biconcave profile. The inner lateral side 15-1 comprises top, middle and bottom inner arcs 17A, 17B, 17C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 17A, 17B, 17C are circular arcs. In the present embodiment the first bottom inner arc 17C extends through an angle greater than 90° (and typically less than 145°). The lower inner arc 17C thereby forms a teardrop feature in the magnet aperture 10-1 external to the first magnet 6-1 . The outer lateral side 16-1 comprises top, middle and bottom outer arcs 18A, 18B, 18C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 18A, 18B, 18C are circular arcs. In the present embodiment the top outer arc 18A extends to form an inverted teardrop feature in the first outer flux barrier 12-1 . The first outer flux barrier 12-1 comprises a concave section adjacent to said top outer arc 18A. The concave section may, for example, be defined by a circular arc joined to said top outer arc 18A.

The middle inner and outer arcs 17B, 18B have at least substantially the same radius and at least substantially the same length. The relative position and orientation of the middle inner arc 17B and the middle outer arc 18B affect the mechanical stress in the first inner bridge 13-1 when the electric machine 1 is operating. The orientation of the middle inner arc 17B is described with reference to an inner virtual chord C1 ; and the orientation of the middle outer arc 18B is described with reference to an outer virtual chord C2. The first inner virtual chord C1 extends between the ends of the middle inner arc 17B; and the first outer virtual chord C2 extends between the ends of the middle outer arc 18B. The relative position and orientation of the first inner and outer virtual chords C1 , C2 are determined by the relative position of the centres of the middle inner and outer arcs 17B, 18B. The first inner and outer virtual chords C1 , C2 are inclined at a first chord line angle a1 relative to each other. The first inner and outer virtual chords C1 , C2 converge towards each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the first chord line angle a1 between the first inner and outer virtual chords C1 , C2 distributes mechanical stress more evenly within the first inner bridge 13-1 . The first chord line angle a1 in the first inner bridge 13-1 is approximately 1 .65° in the present embodiment. A first centreline CL1 of the first inner bridge 13-1 is defined between said first inner and outer virtual chords C1 , C2. The first centreline CL1 is inclined at a first centreline angle relative to the radius R of the rotor 3. The arrangement of the first inner bridge 13-1 is mirrored about the pole axis (d-axis) for the second inner bridge 13-2. As shown in Figure 4A, the first and second intermediate bridges 14-1 , 14-2 are symmetrical about the pole axis (d-axis) and have the same configuration. The configuration of said first intermediate bridge 14-1 will now be described with reference to Figure 4C. The first intermediate bridge 14-1 comprises an inner lateral side 19-1 defined by the intermediate magnet aperture 10-2; and an outer lateral side 20-1 defined by the first inner flux barrier 1 1 - 1 . The inner lateral side 19-1 and the outer lateral side 20-1 are concave such that the first intermediate bridge 14-1 has a biconcave profile. The inner lateral side 19-1 comprises top, middle and bottom inner arcs 21 A, 21 B, 21 C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 21 A, 21 B, 21 C are circular arcs. In the present embodiment the lower inner arc 21 C extends through an angle greater than 90° (and typically less than 145°). The lower inner arc 21 C thereby forms a teardrop feature in the intermediate magnet aperture 10-2 external to the second magnet 6-2. The outer lateral side 20-1 comprises top, middle and bottom outer arcs 22A, 22B, 22C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 22A, 22B, 22C are circular arcs. In the present embodiment the top outer arc 22A extends to form an inverted teardrop feature in the first inner flux barrier 1 1 -1 . The first inner flux barrier 1 1 -1 comprises a concave section adjacent to said top outer arc 22A. The concave section may, for example, be defined by a circular arc joined to said top outer arc 22A.

The second middle inner arc 21 B and the second middle outer arc 22B have at least substantially the same radius and at least substantially the same length. The relative position and orientation of the second middle inner arc 21 B and the second middle outer arc 22B affect the mechanical stress in the first intermediate bridge 14-1 when the electric machine 1 is operating. The orientation of the second middle inner arc 21 B and the second middle outer arc 22B is described herein with reference to respective second inner and outer virtual chords C3, C4. The second inner virtual chord C3 extends between the ends of the second middle inner arc 21 B; and the second outer virtual chord C4 extends between the ends of the second middle outer arc 22B. The relative position and orientation of the second inner and outer virtual chords C3, C4 are determined by the relative position of the centres of the middle inner and outer arcs 21 B, 22B. The second inner and outer virtual chords C3, C4 are inclined at a second chord line angle a2 relative to each other. The second inner and outer virtual chords C3, C4 converge towards each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the second chord line angle a2 between the second inner and outer virtual chords C3, C4 distributes mechanical stress more evenly within the first intermediate bridge 14-1 . The second chord line angle a2 in the first intermediate bridge 14-1 is approximately 0.29° in the present embodiment. A second centreline CL2 of the first intermediate bridge 14-1 is defined between said second inner and outer chords C3, C4. The second centreline CL2 is inclined at a second centreline angle relative to the radius R of the rotor 3. The arrangement of the first intermediate bridge 14-1 is mirrored about the pole axis (d-axis) for the second intermediate bridge 14-2.

The first and second inner bridges 13-1 , 13-2 and the first and second intermediate bridges 14-1 , 14-2 are configured to maintain the mechanical integrity of the laminations of the rotor 3 at high rotational speeds whilst also controlling flux density.

A variant of the rotor 3 shown in Figures 4A-C is shown in Figures 5A and 5B. Like reference numerals are used for like components.

As shown in Figure 5A, the first and second outer bridges 24-1 , 24-2 are formed between the outer magnet aperture 10-3 in the outer layer L3 and the first and second external flux barrier 23-1 , 23-2. The first and second outer bridges 24-1 , 24-2 are symmetrical about the pole axis (d-axis) and have the same configuration. The configuration of the second outer bridge 24-2 will now be described with reference to Figure 5B. The second outer bridge 24-2 comprises an inner lateral side 25-2 defined by the outer magnet aperture 10-3; and an outer lateral side 26-2 defined by the second external groove 23-2. The inner lateral side 25-2 and the outer lateral side 26-2 are concave such that the second outer bridge 24-2 has a biconcave profile. The inner lateral side 25-2 comprises top, middle and bottom inner arcs 27A, 27B, 27C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 27A, 27B, 27C are circular arcs. The outer lateral side 26-2 comprises top, middle and bottom outer arcs 28A, 28B, 28C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 28A, 28B, 28C are circular arcs.

The middle inner arc 27B and the middle outer arc 28B have at least substantially the same radius and at least substantially the same length. The relative position and orientation of the middle inner arc 27B and the middle outer arc 28B affect the mechanical stress in the second outer bridge 24-2 when the electric machine 1 is operating. The orientation of the middle inner arc 27B and the middle outer arc 28B is described herein with reference to respective third inner and outer virtual chords C5, C6. The third inner virtual chord C5 extends between the ends of the middle inner arc 27B; and the third outer virtual chord C6 extends between the ends of the middle outer arc 28B. The relative position and orientation of the third inner and outer virtual chords C5, C6 are determined by the relative position of the centres of the middle inner and outer arcs 27B, 28B. The third inner and outer virtual chords C5, C6 are inclined at a third chord line angle a3 relative to each other. The third inner and outer virtual chords C5, C6 converge towards each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the third chord line angle a3 between the third inner and outer virtual chords C5, C6 distributes mechanical stress more evenly within the second outer bridge 24-2. The third chord line angle a3 in the first intermediate bridge 14-1 is approximately 0.29 ° in the present embodiment. The third chord line angle a3 may be in the range 0° to 10 °. A third centreline CL3 of the third intermediate bridge 24-1 is defined between said third inner and outer chords C5, C6. The third centreline CL3 is inclined at a third centreline angle relative to the radius R of the rotor 3. The arrangement of the first intermediate bridge 24-2 is mirrored about the pole axis (d- axis) for the first intermediate bridge 24-1 . A further embodiment of the rotor 3 in accordance with an aspect of the present invention will now be described with reference to Figures 6A and 6B. Like reference numerals are used for like components. The description of this embodiment will focus on the differences over the embodiment shown in Figures 4A-C.

The rotor 3 comprises six (6) magnet poles 5a-h each comprising three (3) permanent magnets 6-n (where n represents the number of magnets in each of said magnet poles 5a-h) arranged in an outer layer L1 , an intermediate layer L2 and an inner layer L3. The permanent magnets 6-n are mounted in magnet apertures 10-n. The rotor 3 comprises inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 each comprise an air-filled cavity. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are internal apertures (or holes) formed within the rotor 3 and are inset from the outer surface SC1 of the rotor 3.

The outer flux barriers 12-1 , 12-2 are associated with the inner magnet aperture 10-1 in the inner layer L3. First and second inner bridges 13-1 , 13-2 are formed between the inner magnet aperture 10-1 in the inner layer L3 and the outer flux barriers 12-1 , 12-2. The inner flux barriers 1 1 -1 , 1 1 -2 are associated with the intermediate magnet aperture 10-2 in the intermediate layer L2. First and second intermediate bridges 14-1 , 14-2 are formed between the intermediate magnet aperture 10-2 in the intermediate layer L2 and the inner flux barriers 1 1 -1 , 1 1 -2.

As shown in Figure 6A, the first and second inner bridges 13-1 , 13-2 are symmetrical about the pole axis (d-axis) and have the same configuration. The configuration of said first inner bridge 13-1 will now be described with reference to Figure 6B. The first inner bridge 13-1 comprises an inner lateral side 15-1 defined by the inner magnet aperture 10-1 ; and an outer lateral side 16-1 defined by the first outer flux barrier 12-1 . The inner lateral side 15-1 and the outer lateral side 16-1 are concave such that the first inner bridge 13-1 has a biconcave profile. The inner lateral side 15-1 comprises top, middle and bottom inner arcs 17A, 17B, 17C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 17A, 17B, 17C are circular arcs. The outer lateral side 16-1 comprises top, middle and bottom outer arcs 18A, 18B, 18C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 18A, 18B, 18C are circular arcs. In the present embodiment the first bottom outer arc 18C extends through an angle greater than 90° (and less than 180°). The first bottom outer arc 18C forms a teardrop feature in the first outer flux barrier 12-1 .

The middle inner and outer arcs 17B, 18B have at least substantially the same radius and at least substantially the same length. The relative position and orientation of the middle inner arc 17B and the middle outer arc 18B affect the mechanical stress in the first inner bridge

13- 1 when the electric machine 1 is operating. The orientation of the middle inner arc 17B is described with reference to a first inner virtual chord C1 ; and the orientation of the middle outer arc 18B is described with reference to a first outer virtual chord C2. The first inner virtual chord C1 extends between the ends of the middle inner arc 17B; and the first outer virtual chord C2 extends between the ends of the middle outer arc 18B. The relative position and orientation of the first inner and outer virtual chords C1 , C2 are determined by the relative position of the centres of the middle inner and outer arcs 17B, 18B. The first inner and outer virtual chords C1 , C2 are inclined at a first chord line angle a1 relative to each other. The first inner and outer virtual chords C1 , C2 diverge from each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the first chord line angle a1 between the first inner and outer virtual chords C1 , C2 distributes mechanical stress more evenly within the first inner bridge 13-1 . The first chord line angle a1 in the first inner bridge 13-1 is approximately 3.09 ° in the present embodiment. A first centreline CL1 of the first inner bridge 13-1 is defined between said first virtual inner and outer chords C1 , C2. The first centreline CL1 is inclined at a first centreline angle relative to the radius R of the rotor 3. The arrangement of the first inner bridge 13-1 is mirrored about the pole axis (d-axis) for the second inner bridge 13-2. The arrangement of the first and second intermediate bridges 14-1 , 14-2 is unchanged from those described herein with reference to Figures 4A-C.

The first and second inner bridges 13-1 , 13-2 and the first and second intermediate bridges

14- 1 , 14-2 are configured to maintain the mechanical integrity of the laminations of the rotor 3 at high rotational speeds whilst also controlling flux density.

A further embodiment of a rotor 3 in accordance with an aspect of the present invention will now be described with reference to Figures 7A and 7B. Like reference numerals are used for like components. The description of this embodiment will focus on the differences over the embodiment shown in Figures 3A-C. The rotor 3 in this embodiment comprises eight (8) magnet poles 5 each having the same configuration. Only a first magnet pole 5a is shown in Figures 8A-8C. Each magnet poles 5-n in the rotor 3 is symmetrical about the d-axis and the description herein focuses on the configuration of the features on one side thereof. The rotor 3 is for use in a permanent magnet synchronous motor. The rotor 3 is made up of a plurality of laminations of a ferromagnetic material to form a rotor iron. The rotor 3 is disposed in a stator having a plurality of coil windings. The rotor 3 comprises eight (8) magnet poles 5a-h each comprising three (3) permanent magnets 6-n (where n represents the number of magnets in each of said magnet poles 5a-h). In alternate embodiments, the rotor 3 may comprise less than or more than eight (8) magnet poles 5a-h. Moreover, each magnet pole 5a-h may comprise less than or more than three (3) magnets 6-n. In the present embodiment, each magnet 6-n has a longitudinal centreline which extends in a substantially tangential direction (i.e. perpendicular to a radius of the rotor 3). The permanent magnets 6-n generate a magnetic flux and a torque is generated to drive the rotor 3 by energising coil windings in a stator. In use, the rotor 3 rotates about a rotational axis Z (extending perpendicular to the plane of the page in Figure 7A).

The arrangement of the magnet poles 5a-h is the same as the arrangement illustrated in Figure 3A. The magnet poles 5a-h are angularly separated from each other and an inter-pole region 7a-h is formed between adjacent magnet poles 5a-h. The magnet poles 5a-h each extend radially outwardly from the rotational axis Z of the rotor 3. The rotor 3 has an outer surface SC1 in the form of a circular cylinder. The magnet poles 5a-h all have the same general configuration and, for the sake of brevity, only a first magnet pole 5a will be described herein. The reference frame comprises a pole axis (d-axis) aligned to the permanent magnet flux of the first magnet pole 5a. An inter-pole axis (q-axis) arranged transverse to the direction of the first magnet pole 5a (i.e. transverse to the pole axis (d- axis)) forms a centre-line of the inter-pole regions 7a-h. The rotor 3 in the present embodiment comprises eight (8) magnet poles and the angular separation of the d-axes of adjacent magnet poles 5a-h is 45°. The angular separation of the pole axis (d-axis) of the first magnet pole 5a and the inter-pole axis (q-axis) of an adjacent first inter-pole region 7a is 22.5° in the present embodiment. It will be understood that the principles described herein may be implemented in an electric machine 1 having different numbers of magnet poles 5-n. For example, the rotor 3 may comprise ten (10) magnet poles corresponding to an angular separation of 36° between the d-axes of adjacent magnet poles and an angular separation of 18° between the pole axis (d-axis) and the inter-pole axis (q-axis) of an adjacent first inter- pole region 7a. By way of further example, the rotor 3 may comprise six (6) magnet poles corresponding to an angular separation of 60 ° between the d-axes of adjacent magnet poles and an angular separation of 30° between the pole axis (d-axis) and the inter-pole axis (q- axis) of an adjacent first inter-pole region 7a.

The rotor 3 comprises a plurality of like poles 5a-f and the first pole 5a is shown in Figure 7A. The permanent magnets 6-n are each mounted in a respective magnet aperture 10-n formed in the rotor 3. The magnet apertures 10-n are internal apertures which extend substantially parallel to the rotational axis Z. The permanent magnets 6-n in the first magnet pole 5a are arranged in a radially outer layer L1 , an intermediate layer L2 and a radially inner layer L3. As shown in Figure 7A, the outer, intermediate and inner layers L1 -L3 are arranged concentrically about the rotational axis Z of the rotor 3 with a radial offset between each of the outer, intermediate and inner layers L1 -L3. The outer, intermediate and inner layers L1 - L3 within the first pole 5a each comprise one of said permanent magnets 6-n. The permanent magnets 6-n are each arranged substantially perpendicular to the radius R of the rotor 3. In an alternative embodiment, two or more permanent magnets 6-n may be provided in each of the outer, intermediate and inner layers L1 -L3. In alternative embodiments the permanent magnets 6-n may be arranged in only some of said outer, intermediate and inner layers L1 -L3. For example, the magnet poles 5a-h may comprise an outer layer L1 and/or an inner layer L3. In certain embodiments, the permanent magnets 6-n may be arranged in additional layers. The magnet apertures 10-n each comprise first and second lateral stops for inhibiting lateral movement of the permanent magnets 6-n. The first and second lateral stops comprise first and second protrusions for engaging respective sides of the permanent magnet 6-n disposed in said magnet apertures 10-n.

First and second inner flux barriers 1 1 -1 , 1 1 -2; and first and second outer flux barriers 12-1 , 12-2 are associated with each of the magnet poles 5a-h. The terms "inner" and "outer" are used to define the location of the flux barriers relative to the pole axis (d-axis) of the first magnet pole 5a. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are provided to help ensure the appropriate flux density distribution at the lateral boundaries of the magnet poles 5a-h. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12- 2 are arranged symmetrically about the pole axis (d-axis). The inner flux barriers 1 1 -1 , 1 1 -2 are disposed on opposing sides of the permanent magnet 6-2 in the intermediate layer L2; and the outer flux barriers 12-1 , 12-2 are disposed on opposing sides of the permanent magnet 6-1 in the inner layer L3. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 each comprise an air-filled cavity elongated in a radial direction from the centre of the rotor 3. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are internal apertures (or holes) formed within the rotor 3 and are inset from the outer surface SC1 of the rotor 3. The inner flux barriers 1 1 -1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 are surrounded by the ferromagnetic material of the rotor 3. Thus, the inner flux barriers 1 1 - 1 , 1 1 -2 and the outer flux barriers 12-1 , 12-2 have a closed perimeter.

In the present embodiment the first magnet pole 5a comprises first and second interposed flux barriers 30-1 , 30-2. As shown in Figure 7A, the first and second interposed flux barriers 30-1 , 30-2 each comprise a void. In the present embodiment, the first and second interposed flux barriers 30-1 , 30-2 each have a generally teardrop profile. The first and second interposed flux barriers 30-1 , 30-2 are interposed between the inner magnet aperture 10-1 and the first and second outer flux barriers 12-1 , 12-2. The first and second interposed flux barriers 30-1 , 30-2 each have a major axis which is inclined at an angle of approximately 22.5° relative to a radius of the rotor 3. The first and second interposed flux barriers 30-1 are formed on opposing sides of the inner magnet aperture 10-1 between the inner lateral side 15-1 of the inner magnet aperture 10-1 and the outer lateral side 16-1 of the first outer flux barrier 12-1 . The interposed flux barriers 30-1 , 30-2 are surrounded by the ferromagnetic material of the rotor 3. Thus, the interposed flux barriers 30-1 , 30-2 have a closed perimeter. The first and second interposed flux barriers 30-1 effectively divide each of the first and second inner bridges 13-1 , 13-2 of the embodiment shown in Figures 3A-3C along their length. Thus, the first magnet pole 5a comprises first, second, third and fourth inner bridges 13-1 to 13-4. The first and second inner bridges 13-1 , 13-2 are disposed on a first side of the inner magnet aperture 10-1 ; and the third and fourth inner bridges 13-3, 13-4 are disposed on a second side of the inner magnet aperture 10-1 . Each of the first, second, third and fourth inner bridges 13-1 to 13-4 in the present embodiment is narrower than the first and second inner bridges 13-1 , 13-2 in the previous embodiments. The intermediate and outer magnet apertures 10-2, 10-3 (disposed in the intermediate layer L2 and the outer layer L3 respectively), and the inner flux barriers 1 1 -1 , 1 1 -2 are substantially unchanged from the arrangement described herein with reference to Figures 3A-3C.

The configuration of the third and fourth inner bridges 13-3, 13-4 will now be described with reference to Figure 7B. The third inner bridge 13-3 comprises an inner lateral side 31 -3 defined by the inner magnet aperture 10-1 ; and an outer lateral side 32-3 defined by the second interposed flux barrier 30-2. The inner lateral side 31 -3 and the outer lateral side 32- 3 form opposing sides of the third inner bridge 13-3 and are concave such that the third inner bridge 13-3 has a biconcave profile. The inner lateral side 31 -3 comprises top, middle and bottom inner arcs 33A, 33B, 33C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 33A, 33B, 33C are circular arcs. The outer lateral side 32- 3 comprises top, middle and bottom outer arcs 34A, 34B, 34C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 34A, 34B, 34C are circular arcs. In the present embodiment the top inner arc 34A forms an inverted teardrop feature in the inner magnet aperture 10-1 . The middle inner and outer arcs 33B, 34B oppose each other and have at least substantially the same radius. The radius of the top inner arc 33A is 0.63mm, the middle inner arc 33B is 6mm ; and the lower inner arc 33C is 0.53mm. The radius of the top outer arc 34A is 0.7mm; the radius of the middle outer arc 34B is 7mm; and the radius of the lower outer arc 34C is 1 .5mm. A first inner virtual chord C1 extends between the ends of the middle inner arc 33B; and a first outer virtual chord C2 extends between the ends of the middle outer arc 34B. The first inner and outer virtual chords C1 , C2 are inclined at a first chord line angle a1 relative to each other. The first inner and outer virtual chords C1 , C2 converge towards each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the first chord line angle a1 distributes mechanical stress more evenly within the third bridge 13- 3. The first chord line angle a1 in the third inner bridge 13-3 is approximately 12 ° in the present embodiment. The first inner virtual chord C1 is oriented at an inner chord angle β relative to a longitudinal centreline of the inner magnet 6-1 . The inner chord angle β in the present embodiment is approximately 93°.

The fourth inner bridge 13-4 comprises an inner lateral side 35-4 defined by the second interposed flux barrier 30-2 and an outer lateral side 36-4 defined by the second outer flux barrier 12-2. The inner lateral side 35-4 and the outer lateral side 36-4 form opposing sides of the fourth inner bridge 13-4 and are concave such that the fourth inner bridge 13-4 has a biconcave profile. The inner lateral side 35-4 comprises top, middle and bottom inner arcs 37A, 37B, 37C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 37A, 37B, 37C are circular arcs. The outer lateral side 36-4 comprises top, middle and bottom outer arcs 38A, 38B, 38C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 38A, 38B, 38C are circular arcs. In the present embodiment the top outer arc 38A forms an inverted teardrop feature in the second outer flux barrier 12-1 . The middle inner and outer arcs 37B, 37B oppose each other and have at least substantially the same radius. The radius of the top inner arc 37A is 0.7mm; the middle inner arc 37B is 12mm; and the lower inner arc 37C is 1 .5mm. The radius of the top outer arc 38A is 0.7mm; the radius of the middle outer arc 38B is 12mm; and the radius of the lower outer arc 38C is 1 .2mm.

The second interposed flux barrier 30-2 is defined by the outer lateral side 32-3 of the third inner bridge 13-3 and the inner lateral side 35-4 of the fourth inner bridge 13-3. The second interposed flux barrier 30-2 has a substantially continuous outer profile. In the present embodiment, the second interposed flux barrier 30-2 comprises the top, middle and bottom outer arcs 34A, 34B, 34C of the outer lateral side 32-3; and the top, middle and bottom inner arcs 37A, 37B, 37C of the inner lateral side 35-4. The top outer arc 34A and the top inner arc 37A have a common centre and the same radius, thereby combining to form an upper circular arc having a continuous circular profile. The bottom outer arc 34C and the bottom inner arc 37C have a common centre and the same radius, thereby combining to form a lower circular arc having a continuous circular profile. The upper and lower circular arcs form opposing minor sides of the second interposed flux barrier 30-2. A major axis of the second interposed flux barrier 30-2 extends through the centres of the upper and lower continuous circular arcs. The major axis of the second interposed flux barrier 30-2 is oriented at an angle 22° to a radius of the rotor 3. A minor axis of the second interposed flux barrier 30-2 extends perpendicular to the major axis. The middle outer arc 34B and the middle inner arc 37B form opposing major sides of the second interposed flux barrier 30-2.

A second inner virtual chord C3 extends between the ends of the middle inner arc 37B; and a second outer virtual chord C4 extends between the ends of the middle outer arc 38B. The second inner and outer virtual chords C3, C4 are inclined at a second chord line angle a2 relative to each other. The second inner and outer virtual chords C3, C4 converge towards each other in a first direction extending outwardly from the rotor 3. It has been recognised that reducing the second chord line angle a2 distributes mechanical stress more evenly within the third bridge 13-3. The second chord line angle a2 in the fourth inner bridge 13-4 is approximately 2° in the present embodiment. The second outer virtual chord C4 is oriented at an outer chord angle γ relative to the longitudinal centreline of the inner magnet 6-1 . The outer chord angle γ in the present embodiment is approximately 45°.

The first, second, third and fourth inner bridges 13-1 , 13-2, 13-3, 13-4 are configured to maintain the mechanical integrity of the laminations of the rotor 3 at high rotational speeds whilst also controlling flux density. In the present embodiment, there are no interposed flux barriers associated with the intermediate layer L2 or the outer layer L1 . It will be understood that additional interposed flux barriers 30-1 may be provided. For example, interposed flux barriers may be associated with the outer magnet aperture 10-3 in the outer layer L1 . Alternatively, or in addition, interposed flux barriers may be associated with the intermediate magnet aperture 10-2 in the intermediate layer L2.

A variant of the rotor 3 shown in Figures 7A and 7B is shown in Figure 7C. The second interposed flux barrier 30-2 is defined by the outer lateral side 32-3 of the third inner bridge 13-3 and the inner lateral side 35-4 of the fourth inner bridge 13-3. The second interposed flux barrier 30-2 comprises the top, middle and bottom outer arcs 34A, 34B, 34C of the outer lateral side 32-3; and the top, middle and bottom inner arcs 37A, 37B, 37C of the inner lateral side 35-4. The top outer arc 34A and the top inner arc 37A have a common centre and the same radius, thereby combining to form an upper circular arc having a continuous circular profile. The bottom outer arc 34C and the bottom inner arc 37C have a common centre and the same radius, thereby combining to form a lower circular arc having a continuous circular profile. The upper and lower circular arcs form opposing minor sides of the second interposed flux barrier 30-2. A major axis of the second interposed flux barrier 30-2 extends through the centres of the upper and lower continuous circular arcs. The major axis of the second interposed flux barrier 30-2 is oriented at an angle 22 ° to a radius of the rotor 3. The radius of the top inner arc 33A is 0.4mm, the middle inner arc 33B is 7.5mm; and the lower inner arc 33C is 0.5mm. The radius of the top outer arc 34A is 0.6mm; the radius of the middle outer arc 34B is 10mm; and the radius of the lower outer arc 34C is 1 .7mm. The radius of the top inner arc 37A is 0.6mm; the middle inner arc 37B is 12mm; and the lower inner arc 37C is 1 .7mm. The radius of the top outer arc 38A is 0.6mm; the radius of the middle outer arc 38B is 12mm; and the radius of the lower outer arc 38C is 1 .1 mm.

A further embodiment of the rotor 3 is shown in Figures 8A, 8B and 8C. This embodiment is a modification of the embodiment shown in Figures 7A and 7B. Like reference numerals are used for like components in the description of this embodiment. The rotor 3 in this embodiment comprises eight (8) magnet poles 5 each having the same configuration. Only a first magnet pole 5a is shown in Figures 8A-8C. The first magnet pole 5a is symmetrical about a d-axis and the description herein focuses on the configuration of the features on one side thereof. It will be understood that the features on the other side of the first magnet pole 5a are the same.

The third and fourth inner bridges 13-3, 13-4 are shown in Figure 8B. The third inner bridge 13-3 comprises an inner lateral side 31 -3 defined by the inner magnet aperture 10-1 ; and an outer lateral side 32-3 defined by the second interposed flux barrier 30-2. The inner lateral side 31 -3 and the outer lateral side 32-3 form opposing sides of the third inner bridge 13-3 and are concave such that the third inner bridge 13-3 has a biconcave profile. The inner lateral side 31 -3 comprises a middle inner arc 33B and a bottom arc 33C joined to each other to form a continuous inner profile. The middle and bottom inner arcs 33B, 33C are both circular arcs. The outer lateral side 32-3 comprises top, middle and bottom outer arcs 34A, 34B, 34C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 34A, 34B, 34C are circular arcs. The middle inner and outer arcs 33B, 34B oppose each other. The middle inner arc 33B and the middle outer arc 34B have substantially the same radius. In the present embodiment, the middle inner arc 33B has a radius of 20mm; the lower inner arc 33C has a radius of 0.7mm; the top outer arc 34A has a radius of 0.7mm; the middle outer arc 34B has a radius of 20mm; and the lower outer arc 34C has a radius of 0.86mm. It will be understood that the radii of the arcs forming the inner lateral side 31 -3 and the outer lateral side 32-3 may be different in variants. The fourth inner bridge 13-4 comprises an inner lateral side 35-4 defined by the second interposed flux barrier 30-2 and an outer lateral side 36-4 defined by the second outer flux barrier 12-2. The inner lateral side 35-4 and the outer lateral side 36-4 form opposing sides of the fourth inner bridge 13-4 and are concave such that the fourth inner bridge 13-4 has a biconcave profile. The inner lateral side 35-4 comprises top, middle and bottom inner arcs 37A, 37B, 37C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 37A, 37B, 37C are circular arcs. The outer lateral side 36-4 comprises top, middle and bottom outer arcs 38A, 38B, 38C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 38A, 38B, 38C are circular arcs. In the present embodiment the top outer arc 38A forms an inverted teardrop feature in the second outer flux barrier 12-1 . The middle inner and outer arcs 37B, 38B are disposed on opposing sides of the second interposed flux barrier 30-2. In the present embodiment, the top inner arc 37A has a radius of 0.7mm; the middle inner arc 37B has a radius of 16mm; the lower inner arc 37C has a radius of 0.86mm; the top outer arc 38A has a radius of 0.76mm; the middle outer arc 38B has a radius of 18mm; and the lower outer arc 38C has a radius of 1 .1 mm. It will be understood that the radii of the arcs forming the inner lateral side 35-4 and the outer lateral side 36-4 may be different in variants. The second interposed flux barrier 30-2 is defined by the outer lateral side 32-3 of the third inner bridge 13-3 and the inner lateral side 35-4 of the fourth inner bridge 13-4. The second interposed flux barrier 30-2 has a substantially continuous outer profile. In the present embodiment, the second interposed flux barrier 30-2 consists of the top, middle and bottom outer arcs 34A, 34B, 34C of the outer lateral side 32-3; and the top, middle and bottom inner arcs 37A, 37B, 37C of the inner lateral side 35-4. The top outer arc 34A and the top inner arc 37A have a common centre and the same radius, thereby combining to form an upper circular arc having a continuous circular profile. The bottom outer arc 34C and the bottom inner arc 37C have a common centre and the same radius, thereby combining to form a lower circular arc having a continuous circular profile. The upper and lower circular arcs form opposing minor sides of the second interposed flux barrier 30-2. A major axis of the second interposed flux barrier 30-2 extends through the centres of the upper and lower continuous circular arcs. A minor axis of the second interposed flux barrier 30-2 extends perpendicular to the major axis. The middle outer arc 34B and the middle inner arc 37B form opposing major sides of the second interposed flux barrier 30-2. The middle inner arc 37B has a smaller radius than the middle outer arc 34B such that the second interposed flux barrier 30- 2 is not symmetrical about its major axis. With particular reference to Figure 8C, the rotor 3 in this variant comprises third and fourth interposed flux barriers 39-1 , 39-2 associated with the intermediate magnet aperture 10-2 in the intermediate layer L2. The third and fourth interposed flux barriers 39-1 , 39-2 are interposed between the intermediate magnet aperture 10-2 in the intermediate layer L2 and the inner flux barriers 1 1 -1 , 1 1 -2. In this arrangement, first, second, third and fourth intermediate bridges 14-1 , 14-2, 14-3, 14-4 are formed in the intermediate layer L2. The interposed flux barriers 39-1 , 39-2 associated with the intermediate magnet aperture 10-2 in the intermediate layer L2 each comprise a void formed in the rotor 3. The major axis of each interposed flux barriers 39-1 , 39-2 is inclined at an angle of approximately 8.5° relative to a radius of the rotor 3. In variants, the major axis of the interposed flux barriers 39-1 , 39-2 may be inclined at an angle in the range 0° to 45° relative to the radius of the rotor 3. The interposed flux barriers 39-1 , 39-2 are surrounded by the ferromagnetic material of the rotor 3. Thus, the interposed flux barriers 39-1 , 39-2 have a closed perimeter. The third and fourth interposed flux barriers 39-1 , 39-2 form the first, second, third and fourth intermediate bridges 14-1 to 14-4. The first and second intermediate bridges 14-1 , 14-2 are disposed on a first side of the intermediate magnet aperture 10-2; and the third and fourth intermediate bridges 14-3, 14-4 are disposed on a second side of the intermediate magnet aperture 10-2. Each of the first, second, third and fourth intermediate bridges 14-1 to 14-4 in the present embodiment is narrower than the first and second intermediate bridges 14-1 , 14- 2 in the previous embodiments.

The third and fourth intermediate bridges 14-3, 14-4 are shown in Figure 8C. The third intermediate bridge 14-3 comprises an inner lateral side 40-3 defined by the intermediate magnet aperture 10-2; and an outer lateral side 41 -3 defined by the fourth interposed flux barrier 39-2. The inner lateral side 40-3 and the outer lateral side 41 -3 form opposing sides of the third intermediate bridge 14-3 and are concave such that the third intermediate bridge 14-3 has a biconcave profile. The inner lateral side 40-3 comprises a middle inner arc 42B and a bottom arc 42C joined to each other to form a continuous inner profile. The middle and bottom inner arcs 42B, 42C are both circular arcs. The outer lateral side 41 -3 comprises top, middle and bottom outer arcs 43A, 43B, 43C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 43A, 43B, 43C are circular arcs. The middle inner and outer arcs 42B, 43B oppose each other. In the present embodiment, the middle inner arc 42B has a radius of 16.1 mm; the lower inner arc 42C has a radius of 0.56mm; the top outer arc 43A has a radius of 0.34mm; the middle outer arc 43B has a radius of 10.88mm; and the lower outer arc 43C has a radius of 0.34mm. It will be understood that the radii of the arcs forming the inner lateral side 40-3 and the outer lateral side 41 -3 may be different in variants.

The fourth intermediate bridge 14-4 comprises an inner lateral side 44-4 defined by the fourth interposed flux barrier 39-2 and an outer lateral side 45-4 defined by the second outer flux barrier 1 1 -2. The inner lateral side 44-4 and the outer lateral side 45-4 form opposing sides of the fourth intermediate bridge 14-4 and are concave such that the fourth intermediate bridge 14-4 has a biconcave profile. The inner lateral side 44-4 comprises top, middle and bottom inner arcs 46A, 46B, 46C joined to each other to form a continuous inner profile. The top, middle and bottom inner arcs 46A, 46B, 46C are circular arcs. The outer lateral side 45-4 comprises top, middle and bottom outer arcs 47A, 47B, 47C joined to each other to form a continuous outer profile. The top, middle and bottom outer arcs 47A, 47B, 47C are circular arcs. In the present embodiment, the top inner arc 46A has a radius of 0.34mm; the middle inner arc 46B has a radius of 17.84mm; the lower inner arc 46C has a radius of 0.34mm; the top outer arc 47A has a radius of 1 .07mm; the middle outer arc 47B has a radius of 12mm; and the lower outer arc 47C has a radius of 0.64mm. It will be understood that the radii of the arcs forming the inner lateral side 44-4 and the outer lateral side 45-4 may be different in variants. The fourth interposed flux barrier 39-2 is defined by the outer lateral side 41 -3 of the third intermediate bridge 14-3 and the inner lateral side 44-4 of the fourth inner bridge 14-4. The fourth interposed flux barrier 39-2 has a substantially continuous outer profile. In the present embodiment, the fourth interposed flux barrier 39-2 consists of the top, middle and bottom outer arcs 43A, 43B, 43C of the outer lateral side 41 -3; and the top, middle and bottom inner arcs 46A, 46B, 46C of the inner lateral side 44-4. The top outer arc 43A and the top inner arc 46A have a common centre and the same radius, thereby combining to form an upper circular arc having a continuous circular profile. The bottom outer arc 43C and the bottom inner arc 46C have a common centre and the same radius, thereby combining to form a lower circular arc having a continuous circular profile. The upper and lower circular arcs form opposing minor sides of the fourth interposed flux barrier 39-2. A major axis of the fourth interposed flux barrier 39-2 extends through the centres of the upper and lower continuous circular arcs. A minor axis of the fourth interposed flux barrier 39-2 extends perpendicular to the major axis. The middle outer arc 43B and the middle inner arc 46B form opposing major sides of the fourth interposed flux barrier 39-2. The middle inner arc 46B has a larger radius than the middle outer arc 43B such that the fourth interposed flux barrier 39-2 is not symmetrical about its major axis. The fourth interposed flux barrier 39-2 is, however, symmetrical about its minor axis. The rotor 3 also comprises interposed flux barriers 40-1 , 40-2 associated with the outer magnet aperture 10-3 in the outer layer L1 . The interposed flux barriers 40-1 , 40-2 are interposed between the outer magnet aperture 10-3 and the inner flux barriers 1 1 -1 , 1 1 -2. The interposed flux barriers 40-1 , 40-2 comprise an aperture which is open to the external surface SC1 of the rotor 3. Thus, the interposed flux barriers 40-1 , 40-2 have an open perimeter. The major axis of each interposed flux barriers 40-1 , 40-2 is inclined at an angle of approximately 6° relative to a radius of the rotor 3. In variants, the major axis of the interposed flux barriers 40-1 , 40-2 may be inclined at an angle in the range 0° to 45° relative to the radius of the rotor 3.

The terms top, middle and bottom are used herein to differentiate between the inner and outer arcs forming the lateral sides of the bridges 13-1 , 13-2, 14-1 , 14-2 formed in the embodiments of the rotor 3 described herein. These terms are used in relation to the orientation of the first magnet pole 5a illustrated throughout the accompanying figures and are not to be understood as limiting on the scope of protection conferred.

It will be appreciated that various modifications may be made to the embodiment(s) described herein without departing from the scope of the appended claims. For example, the thickness of the bridges 13-1 , 13-2, 14-1 , 14-2 may be adjusted in proportion to their length. The centres of the circular arcs forming the inner and outer middle arcs 17B, 18B may be repositioned to adjust the thickness of the bridges 13-1 , 13-2, 14-1 , 14-2.